Second question, can we manually force the available size ?
In Chrome, Developer Tools can be used to force size via the Emulation options. You can either emulate various devices, if your device is not on the list, manually enter width/height.
To determine actual size that is being used by your device, you can use the Try-it-yourself options of the w3schools screen object documentation
http://www.w3schools.com/jsref/obj_screen.asp. Note difference between width and availWidth, and similarly height and availHeight. Is it 480x800?
So from what you describe, first thing I'd do is run the various try-it-yourself options from the w3schools link to determine size of device, and then in your PC, enter those numbers into Chrome Emulation options and see if result is the same. Try it with both your form and the main gwc-demo application.
Reuben
PS
Have you tried generomobile on such a device?